In the beta test of the upcoming Warlords of Draenor expansion pack AddOns were completely disabled to allow Blizzard to discern between client errors (bugs in their own code) and third-party errors (bugs in addon code). It has been a while since the beta started and in a recent update addons were re-enabled to allow addon authors to make sure their projects will work when Patch 6.0 and Warlords of Draenor go live.
In a pre-Warlords of Draenor patch, there will be a number of updates and changes to functions that affect how User Interface Add-Ons will work. Add-Ons in the Beta client has been enabled and will allow UI Add-On authors will have a chance to work on updating their Add-Ons and test these changes out.
I have not yet received a beta invite for Warlords of Draenor but when I do I will make sure everything will work. Looking at the current list of changes suggests everything will work fine at present, so hopefully any necessary changes will be minimal (although the new C_Timer.After() API is an interest).
Should you run into any problems before I am invited to the beta feel free to contact me on Twitter, or file a bug report on GitHub, WoWinterface or Curse. I read every message and investigate every bug report I receive.